Convergence and divergence of old-field plant communities along experimental nitrogen gradients

In 2 of 3 fields, similarity indices indicated that experimental plots having different resource supply rates diverged in species composition with time. There was some evidence of convergence both within and between fields on plots with similar resource supply rates, but divergence was also common, especially at high rates of nutrient supply. Much of the divergence indicated by similarity indices can be explained by plot-to-plot differences in the initial abundances of 3 dominant species: Agropyron repens, Poa pratensis and Schizachyrium scoparium. High N treatments favored A. repens if it was present, but led to major increases in P. pratensis or S. scoparium if A. repens was absent. Results suggest that plots will converge at high N levels. -from Authors